How can I find the password to www-data group to so I can change directory access rights?
You're doing it wrong (tm) :)
In your terminal window, look at the "command prompt":
username@hostname:~$
What is username
there? You can also try entering whoami
[Enter] to see your current username.
From you description it looks like you entered something like
sudo su www-data
or somehow else opened a console session as www-data user. This user has no rights to use sudo
command, so your attempts at using sudo
fail.
www-data
user also has no password set by default, so it is impossible to log in directly as this user. Which is good. You do not want to give it a password.
You need to open another session as your user (or exit your current www-data
session) and type the commands there:
sudo chown yourusername:www-data /path/to/directory
which will ask you for your login password.
(or, maybe, just move your stuff to /var/www
which already has correct permissions. Then you can add your user to www-data group to get read-write privileges to that directory)